The direct mass spectrometric analysis of solutions, especially those in which the solutes are thermally unstable or involatile, has long presented difficulties. There have been a number of different approaches, as reviewed, for example, by P. J. Arpino and G. Guiochon in Analytical Chemistry, June 1979, vol. 51, p. 683A, and as discussed at the first workshop on liquid chromatography--mass spectrometry, held at Montreux in October 1981, the proceedings of which were published in Journal of Chromatography, 1982, volume 251 pps. 91-225. Amongst the many different approaches that have been used, two related techniques, known as electrohydrodynamic ionisation and electrospray ionisation, respectively, will be discussed here in greater detail because of their relevance to the present invention.
In the technique of electrohydrodynamic ionization, which is fully described by B. A. Stimpson and C. A. Evans Jnr. in Journal of Electrostatics, 1978, volume 5 p. 411, and Journal of Physical Chemistry, 1978, volume 82, p. 660, the solution is introduced into the vacuum system of the mass spectrometer through a capillary tube which is charged at high voltage relative to an extractor electrode situated immediately in front of it. This electrode is usually a thin disc with a hole in the centre, and the capillary tube is positioned concentric with the hole and so that its end is situated within the thickness of the disc. The solution to be analysed is ejected into the vacuum system through the capillary by means of a syringe, which is preferably motor driven. A high positive voltage (if positive ions are to be formed) is applied to the capillary, and the syringe plunger compressed to eject liquid into the vacuum system. If the correct conditions are employed (described below), electrohydrodynamic ionisation of the liquid will take place, and a beam of ions characteristic of the solute will be formed, which can be focussed into a conventional mass analyser. In general, it is necessary to use a solvent which has a low volatility (to ensure that the pressure in the vacuum system does not rise too high), and one which is strongly polar and has a reasonably high electrical conductivity. Glycerol with sodium iodide dissolved in it is frequently employed. These requirements are thought to be due to the fact that in electrohydrodynamic ionisation the electrical field does not actually ionise the solute molecules, but merely distorts the forces present at the surface of the liquid to such an extent that ions already present in the solution are directly emitted into the gas phase. These ions are then focussed into the mass spectrometer. Consequently it is necessary for the ions to be present in solution before it encounters the electrical field, and the process works best with with polar sample molecules dissolved in strongly polar solvents, or with a liquid metal sample. Another characteristic is that the flow rate of the solution is best kept at a very low level, so that no droplets of liquid emerge from the capillary. The sample ions are then emitted from sites round the tip of the capillary, and the shape of the capillary and its position relative to the extractor electrode have a profound effect on the efficiency of the ionization.
The electrohydrodynamic ionisation mass spectra of organic samples, obtained from glycerol and sodium iodide solvents, consist in general of peaks due to the molecular ion of the solute clustered with a variable number (between 0 and 10) of glycerol molecules and sometimes sodium ions, or in the case of negative ions, iodide ions. There is little fragmentation of the molecular ion, but the spectra are often difficult to interpret because of the formation of the complex clusters containing an unknown number of glycerol molecules. Further, the use of electrohydrodynamic ionisation sources with solvents other than glycerol and sodium iodide, although possible, is not always satisfactory because the degree of ionisation of the sample in the solution is usually lower, and more volatile solvents can give rise to problems of excessive pressure in the vacuum system due to evaporating solvent vapour. This problem can be reduced by using a nozzle skimmer system and an additional pumping stage in a similar way to that described below, but the ionisation process remains of low efficiency and for organic molecules the only really satisfactory results are obtained with glycerol solvents. Consequently the use of electrohydrodynamic ionisation for liquid chromatograpy-- mass spectrometery is restricted.
In contrast with electrohydrodynamic mass spectrometery, electrospray mass spectrometery does not require glycerol and sodium iodide solvents. It is based on work by M. Dole et al, (described, for example, in Journal of Chemical Physics, 1968, volume 49, p. 2240). A solution containing the sample to be ionised is sprayed from a capillary tube into a region containing gas at approximately atmospheric pressure, towards a small orifice in a plate which leads into the vacuum system of the spectrometer. A high electrical potential is applied between the spraying capillary and the walls of the chamber containing the gas (including the plate with the small orifice). A separation device, usually a nozzle skimmer system like that described by Kantrowitz and Gray in the Review of Scientific Instruments, 1951, volume 22, p. 328, is placed between the region of atmospheric pressure and the vacuum system in order to reduce the quantity of gas flowing into the vacuum system, and to produce a better collimated molecular beam.
The principle of operation of the electrospray source is as follows. The sample to be ionised is dissolved in a solvent, preferably a fairly polar one, and the resultant solution is slowly displaced through the capillary into a region of high gas pressure and electrical field, as explained. As the jet of liquid emerges it becomes charged by the strong field, the solvent begins to evaporate and the jet breaks up into a series of small charged droplets. It was originally thought that these droplets would continue to evaporate until a point known as the Rayleigh limit was reached, where the drop would become unstable because of its increasing charge to volume ratio and break up into smaller drops, at least one of which would carry the charge. This process was thought to continue until all the solvent evaporated, leaving only neutral solvent molecules in the gas phase and ions of the solute, usually clustered with a few solvent molecules. However, the present inventors believe that it is unlikely that a droplet could evaporate sufficiently to reach the Rayleigh limit before an ion, usually solvated, would be lost from the charged drop by a process similar to electrohydrodynamic ionisation. Whatever the principle involved, it is clear from the original work of Dole that the electrospray technique produces ions from solutes of very high molecular weights (e.g. 500,000), and as the energy imparted to the ions is low, very little, if any, fragmentation of the ions takes place. It is therefore well suited for the ionisation of thermally unstable molecules, such as those frequently encountered in biochemistry.
Electrospray ionisation differs from electrohydrodynamic ionization chiefly in the fact that in the former the solution is sprayed into a gas at atmospheric pressure, whilst in the latter, liquid is pumped slowly through a capillary which leads into an evacuated region so that most of the solvent evaporates before it leaves the capillary and ionisation takes place largely at the tip of the capillary tube. Electrospray type ion sources have been interfaced with mass spectrometers, and the use of such a combination for liquid chromatography--mass spectrometry is known. A typical system is described in U.S. Pat. No. 4,209,696.
A process which is related to electrospray ionisation has been developed by J. V. Iribarne and B. A. Thompson, and is described in U.S. Pat. No. 4,300,044. In this process, a solution containing the sample to be ionised is sprayed from a capillary into gas at atmospheric pressure, and the resultant jet of liquid is nebulized by means of a jet of compressed air flowing at right angles to the liquid jet. The droplets of liquid formed in this way are then electrically charged by induction from a high voltage electrode placed close to the nebulizing jet of air. This process is carried out in the mouth of a wide bore tube, into which the charged droplets are swept. The gas flow containing the drops is then directed across the surface of a plate containing a small hole which leads into the mass spectrometer analyser, and the ions formed as the droplets evaporate are caused to enter this hole by means of an electric field applied at right angles to the gas flow. A curtain of inert gas (e.g. carbon dioxide) flows between the plate which contains the hole and a second plate situated a short distance behind it. This is at a higher pressure than the gas in the remainder of the source and consequently escapes through the orifice in the first plate into the source region. This curtain gas serves to isolate the mass spectrometer vacuum system from excessive flows of solvent vapour, water vapour and other contaminants, and enables cryopumping to be carried out in the vacuum chamber of the mass spectrometer by ensuring that most of the gas that enters the vacuum chamber is the cryopumpable curtain gas. However, the process described is not a true electrospray source because the droplets are produced by a jet of air and are charged by induction, whereas in the true electrospray source they are produced by the action of an electric field on the jet of liquid, and no additional nebulization or charging is required.
The main disadvantage encountered with the prior art electrospray ionization systems, such as that described, is that, like electrohydrodynamic ionisation, the ions produced are usually clustered with a variable number of solvent molecules, although this number tends to be smaller than in electrohydrodyamic ionisation. It is thought that the clustering arises from the fact that during the last stages of evaporation the droplets are virtually stopped by collisions with the inert gas molecules, and the remaining collisions due to thermal motion of the gas molecules are not usually sufficiently energetic to remove the last remaining solvent molecules clustered round the sample ion. In U.S. Pat. No. 4,209,696, additional desolvation is achieved by accelerating the solvated ions after they emerge into the vacuum system in a region close to the orifice where the gas pressure is still fairly high. The increased energy of the collisions of the gas molecules is then sufficient to remove more of the solvent molecules from the ions. The ions are then decelerated again in a region further from the orifice where the pressure is lower and there is a much smaller probability of reassociation between the solvent molecules and the ions. A very similar process is described in U.S. Pat. No. 4,121,099, which also describes another problem encountered with prior art electrospray ion sources, that is the problem of providing an efficient focussing action which does not impart a significant energy spread to the ions and which would degrade the performance of the mass spectrometer. The patent suggests a possible solution to the problem by the provision of strong focussing fields in a region very close to the orifice in the free jet expansion where the pressure is high enough for collisions between the gas molecules and the ions to limit the amount of energy that can be imparted to the ions, thereby limiting the energy spread that is imparted to the ions during the focussing process. Additional lens elements may also be provided further from the nozzle to achieve the declustering effect discussed above. However, it is not possible to completely separate the focussing and declustering actions, and it is difficult to optimise both features simultaneously.
When a magnetic sector spectrometer is to be used, the potential of the inlet capillary of the electrospray source must in general be maintained at a value at least as great as the accelerating voltage required by the spectrometer, and the focussing problem is worsened because the ions still emerge through the orifice into the low pressure region with a very low kinetic energy. They must therefore be reaccelerated to the energy required by the spectrometer. It is difficult to construct electrostatic lenses which will achieve this without significant loss of transmission efficiency and broadening of the kinetic energy spectrum of the ions, and when the design of the acceleration step is further constrained by the desolvation requirements it becomes even more difficult. There is considerable advantage, therefore, in completely separating the desolvation stage from the focussing stage so that both processes can be optimised independently, and so that the need to accelerate the ions to cause desolvation can be eliminated. It is an object of the present invention, therefore, to provide means for desolvating the ions and effectively controlling the extent of the desolvation, before they leave the region of the source which is maintained substantially at atmospheric pressure.